This invention relates to fluid compositions which are useful for transmitting power in hydraulic systems. Specifically, it relates to functional fluids having a tendency to cause erosion of hydraulic systems and a newly discovered means of controlling such erosion.
Organic phosphate ester-based functional fluids have been recognized for some time as advantageous for use as the power transmission medium in hydraulic systems. Such systems include recoil mechanisms, fluid-drive power transmissions, and aircraft hydraulic systems. In the latter, phosphate ester-based fluids find particular utility because of their special properties which include high viscosity index, low pour point, high lubricity, low toxicity, low density and low flammability. Thus, for some years, numerous types of aircraft, particularly commercial jet aircraft, have used phosphate ester-based fluids in their hydraulic systems. Other power transmission fluids which have been utilized in hydraulic systems include major or minor amounts of hydrocarbon oils, amides of phosphoric acid, silicate esters, silicones, and polyphenyl ethers.
Additives which perform special functions such as hydrolysis inhibition, viscosity index improvement and foam inhibition are also frequently present in hydraulic fluid. For example, epoxides are utilized commonly in phosphate ester-based hydraulic fluids to inhibit hydrolysis of the phosphate ester.
The hydraulic systems of a typical modern aircraft contain a fluid reservoir, fluid lines and numerous hydraulic valves which actuate various moving parts of the aircraft such as the wing flaps, ailerons, rudder and landing gear. In order to function as precise control mechanisms, these valves often contain passages or orifices having clearances on the order of a few thousandths of an inch or less through which the hydraulic fluid must pass. In a number of instances, valve orifices have been found to be substantially eroded by the flow of hydraulic fluid. Erosion increases the size of the passage and reduces below tolerable limits the ability of the valve to serve as a precision control device. Many aircraft have experienced sagging wing flaps during landings and takeoffs as a result of valve erosion. Thus, a need exists for functional fluid additives which prevent or inhibit the erosion of hydraulic system valves.
Early investigations indicated that valve erosion was caused by cavitation in the fluid as the fluid passed at high velocity from the high-pressure to the low-pressure side of the valve. Efforts to control hydraulic valve erosion by treating the problem as one of cavitation in the fluid are described in Hampton, "The Problem of Cavitation Erosion in Aircraft Hydraulic Systems", Aircraft Engineering, XXXVIII, No. 12 (December, 1966).
Subsequent studies determined that certain valve erosions are associated with the electrokinetic streaming current induced by high-velocity fluid flow. Studies which attribute valve erosion to the streaming current induced by fluid flow include Beck et al., "Corrosion of Servovalves by an Electrokinetic Streaming Current", Boeing Scientific Research Document D1-82-0839 (September, 1969) and Beck et al., "Wear of Small Orifice by Streaming Current Driven Corrosion", Transactions of the ASME, Journal of Basic Engineering, pages 782-791 (December, 1970).
The rate of valve erosion in aircraft hydraulic system valves has been found to vary with the electrical streaming potential of the hydraulic fluid passing through the valve. Streaming potential is defined on pages 4-30 of the Electrical Engineers Handbook, by Pender and Del Mar (New York, Wiley, 1949) as the electromotive force (EMF) created when a liquid is forced by pressure through an orifice. Streaming potential is a function of several factors including the electrical properties and viscosity of the liquid, the applied pressure, and the physical characteristics of the orifice.
A number of methods are disclosed in the patent literature for reducing or inhibiting valve erosion in hydraulic systems. U.S. Pat. No. 2,470,792, issued May 24, 1949 to Schlessinger et al., discloses noncorrosive hydraulic fluid compositions comprising a major amount of an alkyl phosphate ester, a liquid aliphatic ketone and up to 2 percent by weight of water. This patent does not teach the use of any type of soluble salt to control valve erosion.
U.S. Pat. No. 3,352,780, issued Nov. 14, 1987 to Groslambert et al., discloses the use of alkaline earth metal sulfonates, such as calcium sulfonate, as rust inhibitors in phosphate ester-based hydraulic fluids.
U.S. Pat. No. 3,411,923, issued Nov. 19, 1968 to Bretz, discloses a composition comprising (a) a metal-containing organic phosphate complex prepared by reaction of a polyvalent metal salt of an acid phosphate ester with an organic epoxide and (b) a basic alkali or alkaline earth metal salt of a sulfonic or carboxylic acid having at least about 12 aliphatic carbon atoms. This patent teaches that the disclosed composition can be used to inhibit corrosion of metal surfaces. In column 13, lines 55-70, it is also disclosed that these compositions may be useful in hydraulic oils, among other fluids.
U.S. Pat. No. 3,597,359, issued Aug. 3, 1971 to Smith, discloses a functional fluid composition consisting of a hydrocarbon phosphorus ester and a perfluoro alkylene ether compound. This patent teaches that the disclosed compositions inhibit and control damage to mechanical members when the composition is used as a hydraulic fluid.
U.S. Pat. No. 3,679,587, issued Jul. 25, 1972 to Smith, discloses phosphate ester-based functional fluid compositions containing small amounts of perfluorinated anionic surfactants. When employed as a hydraulic fluid, these compositions are taught to have an increased ability to inhibit erosion of the hydraulic system.
U.S. Pat. No. 3,707,501, issued Dec. 26, 1972 to Gentit, et al., discloses hydraulic fluid compositions containing a minor percentage of a quaternary phosphonium compound. This patent teaches that these quaternary phosphonium compounds inhibit damage to the metallic environment containing the hydraulic fluid.
U.S. Pat. No. 3,907,697, issued Sep. 23, 1975 to Burrous, discloses functional fluid compositions containing a small amount of a soluble salt of a perhalometallic or perhalometalloidic acid. The incorporation of the salt into the fluid base is taught to improve the anti-erosion properties of the functional fluid. U.S. Pat. No. 4,206,067, issued Jun. 3, 1980 to MacKinnon, teaches that the functional fluid composition described in U.S. Pat. No. 3,907,697 can be stabilized at elevated temperatures by addition of a high-boiling-point organic base to the fluid.
U.S. Pat. No. 4,252,662, issued Feb. 24, 1981 to Marolewski, et al., discloses a functional fluid composition comprising a minor percentage of an ammonium salt of a phosphorous acid in a phosphorus ester and/or amide-containing base stock. The functional fluid composition is taught to inhibit and control damage to mechanical members in contact with the fluid.
U.S. Pat. No. 4,302,346, issued Nov. 24, 1981 to MacKinnon, discloses a phosphate ester-based functional fluid composition comprising a major amount of a phosphate ester and a perfluorinated anionic surfactant selected from the group consisting of the di- and trivalent metal salts of a perfluoroalkane sulfonic acid or perfluoroalkane disulfonic acid. This patent teaches that the addition of a small amount to the metal salt to the functional fluid greatly enhances the anti-erosion properties of the fluid. Similarly, U.S. Pat. No. 4,324,674, issued Apr. 13, 1982 to MacKinnon, discloses a phosphate ester-based functional fluid containing the amine salts of the above acids and teaches that these salts similarly enhance the anti-erosion properties of the fluid.
Among the additives cited above, the salts of super acids, such as perfluoroalkane sulfonic acids, perfluoroalkane disulfonic acids, perhalometallic acids, and perhalometalloidic acids, have proven to be especially effective as streaming potential inhibitors for phosphate ester-based hydraulic fluids. These salts however are expensive and not widely available. More importantly, recent information suggests that current hydraulic fluids containing these additives are not able to withstand the high service temperatures found in some modern aircraft.
The engine-driven hydraulic pumps in some modern aircraft are mounted on the core of the jet engine. Fluid temperatures as high as 300.degree. F. have been measured in these critical areas. Exposure of hydraulic fluid to these high temperatures has been found to reduce the effective life of the fluid. This high temperature instability is believe to be due to hydrolysis of the phosphate ester component. Epoxides are normally present in the hydraulic fluid to serve as hydrolysis inhibitors. However, at high operating temperatures in the presence of certain commercial streaming potential inhibitors, i.e. the salts of extremely strong acids, the epoxide is believed to be consumed in unproductive side reactions such as polymerization and etherification. Thus, excessive hydrolysis of the phosphate ester fluid occurs resulting in the need to frequently replace the hydraulic fluid. High material and labor costs make frequent replacement of aircraft hydraulic fluid economical undesirable. Therefore, a need exists for an effective streaming potential inhibitor which is compatible with the epoxide component of the hydraulic: fluid under the high operating temperatures found in some modern aircraft.